Research On Mechanical Response And Crack Propagation Of Oil Well Cement Paste Under Load

The main purpose of cementing is to seal reservoirs,gas reservoirs and water layers in wellbore and prevent them from crossing each other.Various static and dynamic mechanical properties of cement stone determine the bearing capacity of cement sheath in bottom hole service,which is an important basis for judging cement sheath cementation and sealing quality.At present,the strength test of cement paste is limited to static load,but the dynamic impact of cement paste in bottom hole service is not taken into account.The impact of casing and bit on cement stone during casing running and drilling will also have different effects on cement stone.These dynamic load impacts will destroy the structure of cement sheath itself.Cement stone is a kind of heterogeneous brittle material.It is easy to crack because of brittleness under load.How to accurately and comprehensively evaluate the mechanical strength of cement paste has an important influence on the correct understanding of various mechanical properties of cement paste.At present,the evaluation of mechanical properties of cement paste materials is limited to static load.The commonly used strength testing instruments in the laboratory are static load with slow loading.There is little research on the strength and crack propagation law of cement paste under dynamic load(SHPB),so it is necessary to study it.Firstly,taking the strength and basic properties of cement paste under different toughening materials,curing temperatures and curing cycles as the initial screening index,the different formulations of oil well cement paste were studied,and the initial screening temperature was 60 degrees,curing cycle was 14 days.The next dynamic and static load comparison test was carried out under the optimal dosage of cement paste.Secondly,the mechanical response mechanism of three different formulations of cement paste under dynamic and static load comparison and the mechanical damage mechanism of different whisker cement paste under dynamic load are studied.The results show that under dynamic load,the strength of three kinds of cement pastes increases,but the toughening mechanism is not obvious;the strength of different whisker cement pastes varies greatly under dynamic load,which is related to the whisker toughening materials contained in them.The cellulose cement paste with the best toughening performance under static load has the lowest strength under dynamic load.Finally,the changes of stress and strain,time stress,fracture form and micro-fracture morphology of cellulose and carbon fibre cement paste under dynamic loading(water-solid ratio 0.44,toughening fiber content 0 wt%-0.5wt%)and modified cellulose and carbon fibre cement paste were explored.The dynamic fracture law and mechanism of toughened cement paste were analyzed by means of dynamic impact and SEM.The results show that the optimum dosage of cellulose and modified cellulose under dynamic impact is different,the optimum dosage of cellulose is 0.3%,the optimum dosage of modified fiber is 0.2%,and the optimum dosage of carbon fiber and modified carbon fiber under dynamic impact is 0.3%.The internal toughening mechanism of cement paste containing fiber toughening materials is different under dynamic and static loads.Under static loading,the main toughening mechanisms of toughened fibers are crack bending,deflection,fiber debonding,fiber pull-out and fiber bridging.Under dynamic loading,the bending and deflection characteristics of the fibers are relatively weak,which can be seen from the microscopic scanning electron microscopy that the main toughening mechanisms are pull-out and bridging.